While LCD displays offer a compact, lightweight alternative to CRT monitors, there are many applications for which LCD displays are not satisfactory due to a low level of brightness, or more properly, luminance. The transmissive LCD used in conventional laptop computer displays is a type of backlit display, having a light-providing surface positioned behind the LCD for directing light outwards, towards the LCD. The light-providing surface itself provides illumination that is essentially Lambertian, that is, having an essentially constant luminance over a broad range of angles. With the goal of increasing on-axis and near-axis luminance, a number of brightness enhancement films have been proposed for redirecting a portion of this light having Lambertian distribution toward normal, relative to the display surface, thus providing some measure of reduced angular divergence of light for this illumination. Various proposed solutions for brightness or luminance enhancement for use with LCD displays and with other types of backlit display types have been described.
U.S. Pat. No. 5,592,332 (Nishio et al.) describes the use of two crossed lenticular lens surfaces for adjusting the angular range of light in an LCD display apparatus. U.S. Pat. No. 5,611,611 (Ogino et al.) describes a rear projection display using a combination of Fresnel and lenticular lens sheets for obtaining the desired light divergence and luminance. U.S. Pat. No. 6,111,696 (Allen et al.) describes a brightness enhancement film for a display or lighting fixture. The surface of the film facing the illumination source is smooth; the opposite surface has a series of structures, such as triangular prisms, for redirecting the illumination angle. The film refracts off-axis light to provide a degree of correction for directing light at narrower angles. However, this film design works best for redirecting off-axis light; incident light that is normal to the film surface may be reflected back toward the source, rather than transmitted.
U.S. Pat. No. 5,629,784 (Abileah et al.) describes various embodiments in which a prism sheet is employed for enhancing brightness, contrast ratio, and color uniformity of an LCD display of the reflective type. The brightness enhancement film is arranged with its structured surface facing the source of reflected light for providing improved luminance as well as reduced ambient light effects. Because this component is used with a reflective imaging device, the prism sheet is placed between the viewer and the LCD surface, rather than in the position used for transmissive LCD systems (that is, between the light source and the LCD). U.S. patent application Publication No. 2001/0053075 (Parker et al.) describes various types of surface structures used in light redirection films for LCD displays, including prisms and other structures. U.S. Pat. No. 5,887,964 (Higuchi et al.) describes a transparent prism sheet having extended prism structures along each surface for improved back-light propagation and luminance in an LCD display. However, much of the on-axis light is reflected rather than transmitted with this arrangement. The arrangement is usable only for small, hand-held displays and does not use a Lambertian light source.
U.S. Pat. No. 6,356,391 (Gardiner et al.) describes a pair of optical turning films for redirecting light in an LCD display, using an array of prisms, where the prisms can have different dimensions. U.S. Pat. No. 6,280,063 (Fong et al.) describes a brightness enhancement film with prism structures on one side of the film having blunted or rounded peaks. U.S. Pat. No. 6,277,471 (Tang) describes a brightness enhancement film having a plurality of generally triangular prism structures having curved facets. U.S. Pat. No. 5,917,664 (O'Neill et al.) describes a brightness enhancement film having “soft” cutoff angles in comparison with conventional film types, thereby mitigating the luminance change as viewing angle increases. U.S. Pat. No. 5,839,823 (Hou et al.) describes an illumination system with light recycling for a non-Lambertian source, using an array of microprisms. U.S. Pat. No. 5,396,350 (Beeson et al.) describes a backlight apparatus with light recycling features, employing an array of microprisms in contact with a light source for light redirection in illumination apparatus where heat may be a problem and where a relatively non-uniform light output is acceptable.
FIG. 1 shows one type of prior art solution, a brightness enhancement film 10 for enhancing light provided from a light source 18. Brightness enhancement film 10 has a smooth side 12 facing towards a Light Guiding Plate 14 (LGP) which contains a reflective surface 19, and rows of prismatic structures 16 facing an LCD component 20. This arrangement, as described in U.S. Pat. Nos. 6,111,696 and 5,629,784 (both listed above), and in U.S. Pat. No. 5,944,405 (Takeuchi et al.), generally works well, improving the on-axis luminance by refraction of off-axis light rays and directing a portion of this light closer to the normal optical axis, thereby providing a somewhat collimated illumination. As FIG. 1 shows, off-axis rays R1 are refracted toward normal. It is instructive to note, however, that, due to total internal reflection (TIR), near-axis light ray R3 can be refracted away from normal at a more extreme angle. In addition, on-axis light ray R4 can actually be reflected back toward light guiding plate 14 for diffusion and reflection from reflective surface 19 rather than being directed toward LCD component 20. This refraction of near-axis light and reflection of at least a portion of on-axis light back into light guiding plate 14 acts to adjust illumination luminance with respect to viewing angle, as is described subsequently. By the action of light guiding plate 14 and reflective surface 19, a portion of the light that is reflected back from brightness enhancement film 10 is eventually diffused and again directed outward toward the LCD component at a generally normal angle. There is, of course, some loss of light after multiple reflections, due to inefficiency of reflective surface 19.
The purpose of brightness enhancement film 10, then, is to redirect the light that is provided over a large angular range from light guiding plate 14, so that more of the output light it provides to LCD component 20 is directed toward normal, improving light direction by providing some degree of collimation. By doing this, brightness enhancement film 10 helps to improve display luminance not only when viewed straight-on, i.e. normal to the display surface, but also when viewed from oblique angles.
While it is considered advantageous to enhance on-axis luminance and provide a more uniform light surface, there are additional considerations for providing improved backlight illumination. Off-axis illumination, at incident angles other than normal to the LCD surface, can compromise image quality in a number of ways. The light-angle dependence of the LC device is shown in FIGS. 2A and 2B. In FIG. 2A, light at normal incidence is propagated through a rear polarizer 202, then through a distance d in an LC layer 200, over which its polarization is modulated according to the pixel state. The illumination is then viewed through a front polarizer 204. Off-axis light, as shown in FIG. 2B, passes through the same components, but is modulated over a distance d′, as shown. Depending on the type of LC device and the angle θ, a slightly different optical phase retardation is applied to the off-axis light. Moreover, due to birefringence of LC materials, different indices of refraction apply for light of different polarization states. This behavior can result in color shifts over different viewing angles. In addition, this treatment of off-axis illumination can also degrade contrast due to stray light and reduce the overall grayscale resolution of the LC device. This behavior can be particularly pronounced with conventional Twisted-Nematic TN LCD components.
Optical compensators provide one solution for correcting this difference in handling off-axis light. Referring to FIG. 3, there is shown a display apparatus 100 using a TN LCD device as LC component 20, with supporting compensators 210,212 and polarizers 202, 204. In terms of its structure, compensator 210,212 typically uses an arrangement of discotic LC elements that act to counteract the positive birefringence of light directors in the LC modulator by geometrically mirroring the spatial orientation of a portion of these light directors. In operation, optical compensator 210, 212 provides a compensating negative birefringence to offset the positive birefringence of LCD component 20, shown as a TN LCD modulator in FIG. 3. With conventional TN devices, two compensator 210, 212 films are used, one on each side of the LCD. Using such an arrangement, contrast can be significantly improved over a range of viewing angles.
Since LCD displays were initially introduced, there have been a number of improvements in LC technology. The Vertically Aligned (VA) type of LCD has been shown to provide improved performance over wide viewing angles. Addition of a compensation film to a VA type LCD yields a significant improvement in contrast. For comparison, FIGS. 4A-4D show ISO contrast plots for the following configurations:
FIG. 4A shows an ISO contrast plot for a TN LCD without compensation, with a legend that applies for both FIGS. 4A and 4B;
FIG. 4B shows an ISO contrast plot for a TN LCD with compensation;
FIG. 4C shows an ISO contrast plot for a VA LCD without compensation, with a legend that applies for both FIGS. 4C and 4D;
FIG. 4D shows an ISO contrast plot for a VA LCD with compensation;
More recent types of LCD provide further improvements. The Optically Compensated Birefringence (OCB) LCD, as its name implies, provides a measure of built-in compensation for inherent birefringence, thus not requiring a compensator in many applications. For comparison, FIGS. 5A and 5B show ISO contrast plots for OCB LCDs without compensation and with compensation, respectively.
Another recent development, the In-Plane Switching (IPS) LCD, using a lateral electrical field for each pixel, provides a more uniform directional control of crystal orientation, resulting in reduced viewing angle-related differences in contrast and color. FIGS. 15A and 15B show ISO contrast plots for IPS LCDs without compensation and with compensation, respectively. In FIG. 15B, a first curve 66 indicates a contrast level of 250. A second curve 68 indicates a contrast level of 200. FIG. 17 shows the contrast profile of an IPS LCD.
With the earlier TN and VA types of LCD, some type of compensator film is generally needed in order to improve performance over wide viewing angles, as is shown in FIGS. 4A-4D. With the more recently developed OCB and IPS LCDs, compensator films may still be used; however, with OCB and IPS devices, the performance improvements provided by compensators may be offset by disadvantages of cost and light loss due to these additional films. It would be advantageous to provide improved contrast when using these devices, without requiring a compensation film.
One way to minimize or eliminate the need for a compensation film is to reduce the angle of incident illumination. Reducing the angular divergence of the illumination yields better contrast and color properties of the modulated light. Fully collimated light, having relatively small divergence angle from normal direction in any azimuthal direction, would be ideal. However, while it would be advantageous to provide fully collimated light from any point on light guiding plate 14 (FIG. 1), this proves to be difficult to achieve.
Referring to FIGS. 6A, 6B, and 6C, there are shown perspective, side, and top views respectively of illumination components for an LC display. Two azimuthal directions are defined: x being parallel to light source 18, y being perpendicular. The surface of light guiding plate 14 is the reference x,y plane. Here, light source 18 is a CCFL (Cold-Cathode Fluorescent Light) or similar component, having height dCCFL and length WCCFL.
From considerations of etendue in y, it can be seen that is possible to provide reduced angular divergence along they-direction. In the general case, etendue E is defined using:E=A×Ω  (1)where A is the area over which the beam propagates and Ω is the beam divergence angle. Since etendue should increase through the optical system, the following relationship holds for an apparatus using a light guiding plate:dCCFLθCCFL<=LLGPθLGP  (2)Where θCCFL is the divergence angle along y of the beam from the CCFL and θLGP is the divergence angle along y from the light guiding plate.
In practice, the value of LLGP is much larger than that of dCCFL so that it is possible to devise a backlight design having θCCFL>>θLGP. This relationship implies that the illumination in y is not divergent over broad angles. However this condition does not hold likewise in the x direction. Instead, since WCCFL and WLGP are close in dimension, it would be difficult to provide good divergence reduction along the x-direction. With respect to etendue, a similar relationship to that of equation (2) holds in this case.WCCFLφCCFL≦WLGPφLGP  (3)Where φCCFL is the divergence angle along x of the beam from the CCFL and φLGP is the divergence angle along x from the LGP.
It is difficult to design a backlighting apparatus that would allow φLGP to be much smaller than φCCFL, that is, where good divergence-reduction would be provided if WCCFL and WLGP are close in dimension. This makes it difficult and inefficient to provide illumination that is collimated or, more generally, at reduced divergence with respect to both x and y axes. Thus, because it is difficult to obtain light at reduced divergence along both x- and y-axes, conventional designs typically employ a compensation film or similar compensator component as an aid to contrast improvement.
Thus, it can be seen that there is a need for a backlighting solution that provides illumination at favorable angles for backlit displays, is not significantly compromised with respect to overall light efficiency, and does not require a compensation film.